Contemporary spark ignited internal combustion engines are operated by electronics to control, among other things, emissions of pollutants into the atmosphere. Environmental legislation continually requires stricter limitations on emissions in automotive applications. To reduce automotive emissions in a spark ignited internal combustion engine precise control of combustion air/fuel ratio is necessary. This is usually done by metering a precisely controlled amount of fuel based on a measured or inferred ah charge mass ingested into the engine. Many control schemes currently control fuel but with less accuracy than necessary. Precise control is difficult because of a deposit, and subsequent evaporation of the deposit, of fuel on the walls of an intake manifold and on intake valves of the engine. This phenomena is sometimes referred to as wall-wetting. To achieve accurate control of the fuel delivered for combustion fuel behavior associated with wall-wetting must be accurately compensated.
Wall-wetting behavior is dynamic and has been characterized by two parameters corresponding to a fraction of injected fuel that is deposited into a film or puddle on a backside of the intake valves and the walls of the intake manifold, and a fraction of the fuel film evaporating from the film between one engine cycle and the next. These two parameters vary with engine operating conditions such as engine speed, load, and temperature. These two parameters also vary over time with engine age, engine intake valve deposits and fuel composition, making it difficult to compensate for wall-wetting with consistent accuracy. Furthermore, during nontrivial transients, the wall-wetting parameters can vary rapidly with rapidly varying operating conditions.
Some prior art schemes that attempt to compensate for the above-introduced wall-wetting behavior exhibit a large lean excursion while opening the throttle (acceleration), and a large rich excursion while closing the throttle because they insufficiently compensate for the wall-wetting behavior. Furthermore, some prior art systems overcompensate the transient fuel dynamics causing an excessively rich mixture during acceleration. Both undercompension and overcompensation fuel control errors are due to inaccurate fuel compensation when the engine dynamic parameters differ from predetermined values. In most of these prior art schemes wall-wetting parameters are experimentally mapped as functions of engine speed and engine load and stored in tables for use in controlling an engine. Mapping wall-wetting parameters is a testing intensive and expensive process. The mapping is usually performed on a single prototype engine that may exhibit behavior not representative of every mass-produced engine and is then applied to mass produced engines. Furthermore, the tables are typically constructed for steady-state operating conditions and a warm engine, making these schemes inaccurate for transient and cold engine operating conditions. Often the prior art schemes rely on ad-hoc/experimentally determined temperature correction factors to compensate for temperature effects, with only limited success. Also, with the long term aging effects such as the accumulation of intake valve deposits, the control accuracy and hence the emissions of the engine deteriorate significantly with age. Emissions deterioration as the engine ages is now an important problem since the 1990 amendments to the Clean Air Act increased the emissions durability requirements to 100,000 miles.
Other (adaptive) prior art schemes address the time-varying nature of the wall-wetting dynamics. These prior art schemes often involve nonlinear programming and parameter space search techniques that are prohibitively computationally intensive and relatively slow to converge in a real time application. The best known prior art schemes take about 40 seconds to converge, which is unacceptably long for application in an automotive environment. Furthermore, these prior art schemes rely on steady-state engine operation and do not adjust for fuel behavior on a cycle-by-cycle basis resulting in poor transient behavior. These long convergence times and the inability to adapt on a cycle-by-cycle basis result in an adaptive system that is slow to respond to changing engine dynamics. Slow response to rapidly changing engine dynamics creates tracking errors that result in unacceptable deviations from a stoichiometric air/fuel ratio during engine transients, and increased emissions.
In summary, prior art mapped fuel compensation schemes do not accurately take time varying engine operating conditions such as engine temperature, engine age, engine valve deposits and fuel composition into account. Furthermore, adaptive prior art fuel compensator schemes are computationally intensive and have inaccurate transient behavior. More accurate transient and cold engine fuel control is necessary in order to meet future emissions requirements. Therefore, what is needed is a more accurate fuel compensation approach for a spark ignition engine that automatically adjusts for time varying fuel delivery dynamic behavior due to causes such as engine operating conditions, engine age, and fuel composition without requiring excessive computational resources.